Yes DC is a big market place in the US (8?.. not even in the top 3? but interesting). They play in the same market but they definitely do not have the same fan-base in their own hometown/US or around the world. Hence, their media market is abysmal local/US/Around the world. Also, DC is nothing if you take a global perspective. Only a handful are big/global market teams in professional sports around the world. (Real Madrid, Barcelona, Dallas Cowboys, Yankees, Manchester Utd. Lakers, etc.). The Nats are not in that category, not even close. The population/density that these teams cover is worldwide (worldwide media market). The Natas are not in that category, not even close. The Nats are not a big market team. First, They have to win loyalty in their own hometown. In order to do that, they have to win championships since they are young franchise. It is going to take time.
Let me put this in perspective.
Soccer.
El Atletico de Madrid plays in the most important spain market (Madrid), they have a lot of money but they are not a global/big market team, not even close. On the other hand, look at El Real Madrid, they are definitely a global/big market team. Between them, theres an abysm. They are not in the same category. The same applies for Nats/Skins.
Football.
Steelers do not play in a big market city like DC but they are a global market team. They have a huge fan-base world wide (hence world wide media market).
Basket.
Clippers play in the same city/stadium where Lakers play and they are not even close to be a global market team.
Base.
Marlins already spent a lot of money. Nobody follow them in FLorida. They are trying to gain loyalty, first, in their hometown. Going to take time.
ETc...
BTW I asked for the definition of a market team, no big market city. Still no source.